1. Field of the Invention
This invention relates to a circuit arrangement which is particularly suitable for use in a burglar security device of the type which operates with two Doppler alarm devices, wherein the difference frequency signal of the two Doppler frequency signals produced by the movement of an object is employed as an alarm-triggering signal, and more particularly to such a circuit arrangement in which a produce detector is employed for forming the difference frequency signals, followed by a low pass filter and a threshold value detector.
2. Description of the Prior Art
It is known in the art to employ the Doppler principle to construct burglar security devices. In such devices, a transmitter emits radiation which is reflected by objects which must be considered, in particular, to consist of human beings. The reflected radiation is received and analyzed by a receiver. If the reflection takes place on a stationary object, the frequency of the received radiation generally agrees with the frequency of the transmitted radiation. If, however, the reflection occurs from an object which is moving at least with a speed component in the direction of the transmitter and/or the receiver, that is towards or away from the receiver, a Doppler frequency shift occurs in the received radiation in comparison to the frequency of the transmitted radiation.
Known burglar security devices which, for example, are commercially available employ electromagnetic radiation in the X-band (radio waves). The frequency range of these radiations lies, for example, at 9.5 GHz. Electromagnetic radiation of this kind can be handled relatively easily. It can be produced, for example, with a semiconductor Gunn diode, and the receiver is equipped, for example, with a Schottky diode. However, a device operating in the X-band has a disadvantage which is extremely serious, at least in individual situations, and which is based on the properties of the electromagnetic radiation. Electromagnetic radiation easily passes through walls, and in particular through windows, and will be reflected from a moving object, e.g. a human being, regardless of whether the person is moving in the area protected and monitored by the burglar device; the person may be in an adjoining passageway or outside of the building in the street. This disadvantage has been eliminated, through one technique, by rendering the device in question insensitive to such an extent that unfortunately it is entirely unreliable in monitoring the relevant area.
Burglar security devices which operate, not with radio waves, but with ultrasonic radiation, e.g. in a frequency range of about 40 KHz, are also already commercially available. An advantage of such devices is that, in comparison to devices operating with radio waves, they require a lower technical outlay and are correspondingly less expensive. However, the ultrasonic devices likewise have serious disadvantages. A fundamental disadvantage is that the emitted ultrasonic radiation may be influenced by air currents and suffer attenuation fluctuations. However, it is not possible to exclude turbulent air, in particular in heated rooms. In order to avoid a false alarm, the aforementioned technique was adopted by rendering the device extremely insensitive. For this reason, and for other reasons, ultrasonic devices have been used virtually only for monitoring small areas, such as motor vehicles and mobile homes.
In an earlier application of Walter Heywang, Max Guntersdorfer and Peter Kleinschmidt, Ser. No. 780,806, filed Mar. 24, 1977, a burglar security device was proposed which, in comparison to the prior art, has a high response sensitivity, on the one hand, and, on the other hand, has a high safeguard against false alarms.
The device proposed by Heywang et al, which operates with a transmitter for radiation to be transmitted, with the receiver for receiving the emitted Doppler frequency shifted radiation which has been reflected by a moving object, and with a device which serves to establish and analyze the reception of Doppler frequency shifted radiation, is characterized in that in the device there are provided two transmitting and receiving arms, of which one arm operates with radio waves and the other with ultrasonic waves. The device applies an analysis signal intended to trigger the alarm only when the Doppler frequency shifted radiation is received simultaneously in both arms, and an analysis signal is detected when the Doppler frequency of the two Doppler frequency shifted radiations differ from one another by no more than a frequency degree emitted in accordance with a tolerance width. In this previous proposal, the tolerance width is determined by the selection of the upper cut-off frequency of an output-end low pass filter.
In the Heywang et al application, an ultrasonic transmitter and a radio transmitter emit their respective radiations in an area to be protected. Reflected radiations are received by respective ultrasonic and radio wave receivers which contain demodulators which cause signals to occur at the respective outputs of the receivers which correspond to the Doppler frequency, assuming a Doppler shift has occurred due to a moving object in the protected area. In the example given, the Doppler frequency of the ultrasonic wave is four times the Doppler frequency of the radio wave. In order to normalize this difference, a divider is connected to the output of the ultrasonic receiver and has a division ratio of 4:1. The outputs of the ultrasonic receiving arm and the radio receiving arm are then fed to a product detector which feeds a low pass filter. The upper cut-off frequency of the low pass filter is dimensioned to provide the prescribed tolerance width for frequency comparison of the two Doppler signals and, upon an ideal frequency match, a direct current signal is provided for triggering an alarm. Each of the receivers may have a band pass filter connected to the output thereof for eliminating a response to Doppler velocities which are not of interest.
In another embodiment the two receivers and their band pass filters are provided to feed the relevant Doppler signals to other evaluation apparatus. The band pass filter of the ultrasonic arm is connected, for example, to a phase locked loop (PLL) circuit which has an oscillator feeding a mixer which receives the ultrasonic signal and which is phase locked thereto by way of a loop which includes an amplifier fed by the mixer and connected to control the frequency of the oscillator. The amplifier is also connected to control the frequency of a second oscillator in the radial wave branch, the natural frequencies of the two oscillators differing by a factor n of the predetermined ratio of the Doppler frequencies. The oscillator in the radio wave arm also feeds a mixer which receives the radio Doppler signal, this latter mixer supplying the mixing product of the two frequencies, the normalized ultrasonic frequency and the radio frequency, through a low pass output filter as in the previously discussed embodiment.
In another embodiment disclosed by Heywang et al, a pair of threshold value detectors are connected to the outputs of the respective receivers at the point where the receivers feed the respective band pass filters in order to detect reflections of excessive intensity which could be provided by high intensity radiation, as may be used for the purpose of jamming.
As is apparent from the foregoing, Heywang et al is concerned with the situation in which analysis signals based on extremely rapidly moving objects are increasingly smaller, the greater the speed of the relevant object.